Communication devices such as User Equipments (UE) are also known as e.g. mobile terminals, wireless terminals and/or mobile stations. User equipments are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
User equipments may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The user equipments in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another user equipment or a server. The concept of user equipment also comprises devices with communication capability of machine-type character such as sensors, measurement devices etc that not necessarily is in any interaction with a user.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “base station”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site.
LTE Mobility
Mobility management is a challenging task in cellular communications systems and a well functioning mobility performance is crucial to the quality experienced by the end user. The Radio Resource Control protocol, RRC, see 3GPP TS 36.331, is the main signaling protocol for configuring, re-configuring and general connection handling in the LTE radio access network (E-UTRAN). RRC controls many functions such as connection setup, mobility, measurements, radio link failure and connection recovery.
A user equipment, UE, in LTE can be in two RRC states: RRC_CONNECTED and RRC_IDLE. In RRC_CONNECTED state, mobility is network-controlled based on e.g. measurements provided by the user equipment. That is, the network decides when and to which cell a user equipment should be handed over, based on e.g. measurement reports provided by the user equipment. The network, i.e. the LTE radio base station, called eNB in E-UTRAN configures various measurement events, thresholds etc based on which the user equipment then sends reports to the network, such that the network can make a decision to hand over the user equipment to a stronger cell as the user equipment moves away from the present cell.
FIG. 1 illustrates a simplified signaling scheme for the LTE handover, HO, procedure. It should be noted that the HO command is in fact prepared in the Target eNB, i.e. the eNodeB that the user equipment will be handed over to, but the message is transmitted via the Source eNB. That is, from the user equipment's perspective the message comes from the Source eNB.
In RRC_IDLE, mobility is handled by UE-based cell-selection, where a nomadic user equipment selects the “best” cell to camp on, based e.g. on various specified criteria and parameters that are broadcasted in the cells. For example, various cells or frequency layers could be prioritized over other, such that the user equipment tries to camp on a particular cell as long as the measured quality of a beacon or pilot in that cell is a threshold better than some other beacon or pilot received from other cells.
The present disclosure is primarily focusing on problems associated with network-controlled mobility as described above, i.e. for an LTE user equipment in RRC_CONNECTED state. The problems associated with failing handovers are therefore described in further detail below.
In a regular situation, and when a RRC_CONNECTED user equipment is moving out from the coverage of a first cell, also called source cell, it should be handed over to a neighboring cell, also called target cell or second cell before loosing the connection to the first cell. That is, it is desirable that the connection is maintained with no or minimal disruption throughout the handover, such that the end-user is unaware of the ongoing handover. In order to succeed with this, it is necessary that                the measurement report that indicates the need for mobility is transmitted by the user equipment and received by the Source eNB, and        the Source eNB has sufficient time to prepare the handover to the target cell (by, among other things, requesting a handover from the Target eNB controlling the target cell), and        the user equipment receives the handover command message from the network, as prepared by the Target eNB in control of the target cell and sent via the source cell to the user equipment, see FIG. 1.        
In addition, and in order for the handover to be successful, the user equipment must finally succeed in establishing a connection to the target cell, which in LTE requires a successful random access request in the target cell, and a subsequent transmission of a HO complete message from the user equipment to the Target eNB. It should be noted that specifications may differ somewhat in the naming of messages.
Thus, it is clear that in order for the handover to succeed, it is necessary that the sequence of events leading to a successful handover is started sufficiently early, so that the radio link to the first cell over which this signaling takes place does not deteriorate too much before completion of the signaling. If such deterioration happens before the handover signaling is completed in the source cell (i.e. first cell), then the handover is likely to fail. Such handover failures are clearly not desirable. The current RRC specification therefore provides various triggers, timers, and thresholds in order to adequately configure measurements, such that the need for handovers can be detected reliably, and sufficiently early.
In FIG. 1, the exemplified measurement report is triggered by a measurement event called A3 event, which in short means that a neighbor cell is found to be an offset better than the current serving cell. This means that a measurement report is sent to the network when a criterion or criteria associated with the event is satisfied. There exists many different measurement event types, and it should be noted that there are multiple events that could be configured to trigger a report.
A network node in control of a cell, such as an eNodeB in LTE terminology maintains a neighbor cell relation list. Whenever a reference is made to a neighbor cell in this disclosure, it should be understood as a reference to a cell typically comprised in the neighbor cell relation list of a network node. A neighbor cell is thus a cell that is often a candidate for a handover. In some cases, the network node maintains a neighbor cell list related to each cell that it is controlling. From the perspective of the user equipment, a neighbor cell is a cell in the proximity or overlapping with the cell to which the user equipment is currently connected to.
Radio Link Failure and RRC Connection Re-Establishment
It may occur that a user equipment looses coverage to the cell that the user equipment is currently connected to. This could occur in a situation when a user equipment enters a fading dip, or that a handover was needed as described above, but the handover failed for one or another reason. This is particularly true if the “handover region” is very short, as will be further described below.
The quality of the radio link is typically monitored in the user equipment e.g. on the physical layer, as described in 3GPP TS 36.300, TS 36.331 and TS 36.133, and summarized below.
Upon detection that the physical layer experiences problems according to criteria defined in TS 36.133, the physical layer sends an indication to the RRC protocol of the detected problems called out-of-sync indication. After a configurable number, N310, of such consecutive indications, a timer, T310, is started. If the link quality is not improved (recovered) while T310 is running, i.e. there are no N311 consecutive “in-sync” indications from the physical layer, a radio link failure, RLF, is declared in the user equipment, see FIG. 2.
The currently relevant timers and counters described above are listed in FIG. 4 for reference. The user equipment may read the timer values and counter constants from system information broadcasted in the cell. Alternatively, it is possible to configure the user equipment with UE-specific values of the timers and counter constants using dedicated signaling, i.e. where specific values and constants are given to specific user equipments with messages directed only to one or more specific user equipment.
The function of the timers and counters used for monitoring radio link failure in LTE is presented in the tables in FIG. 3.
If T310 expires, the user equipment initiates a connection re-establishment to recover the ongoing RRC connection. This procedure includes cell selection by the user equipment. That is, the RRC_CONNECTED user equipment shall try to autonomously find a better cell to connect to, since the connection to the previous cell failed according to the described measurements. It could occur that the user equipment returns to the first cell anyway, but the same procedure is also then executed. Once a suitable cell is selected as further described e.g. in 3GPP TS 36.304, the user equipment requests to re-establish the connection in the selected cell. It is important to note the difference in mobility behaviour as an RLF results in user equipment based cell selection, in contrast to the normally applied network-controlled mobility.
If the re-establishment is successful, which depends on, among other things, if the selected cell and the eNB controlling that cell was prepared to maintain the connection to the user equipment, which implies that is was prepared to accept the re-establishment request, then the connection between the user equipment and the eNB can resume. In LTE, a re-establishment procedure includes a random-access request in the selected cell, followed by higher layer signalling where the user equipment sends a message with content based on which the user equipment can be identified and authenticated. This is needed so that the network can trust that it knows exactly which user equipment is attempting to perform the re-establishment.
A failure of a re-establishment means that the user equipment goes to RRC_IDLE and the connection is released. To continue communication, a new RRC connection has then to be requested and established. A failure could occur e.g. if the eNB that receives the re-establishment request is not able to identify the user equipment that requests the re-establishment. Such a condition may occur if the receiving eNB has not been informed or prepared for a possible re-establishment from this user equipment.
The reason for introducing the timers T31x and counters N31x described above is to add some freedom and hysteresis for configuring the criteria for when a radio link should be considered as failed and need to be re-established. This is desirable, since it would affect the end-user performance negatively if a connection is abandoned prematurely if it turned out that the loss of link quality was temporary and the user equipment succeeded in recovering the connection without any further actions or procedures, e.g. before T310 expires, or before the counter reaches value N310.
The re-establishment procedure will be described in the following:
The network node, such as eNB, controlling a target cell receives a recovery request message from the user equipment, such as an RRC connection re-establishment request. In response to this message, the target eNB may respond with an RRC connection re-establishment message sent to the user equipment, by which the target eNB accepts the re-establishment request. The message may include various configuration parameters, such that the connection can be adapted and continued in the new cell. Other message names may of course apply, such as any reference to cell re-selection or handover.
Upon reception of re-establishment message, the user equipment may now process the content of that message, and resume the RRC connection according to the content and commands provided therein. Typically, the user equipment would further send a confirm message to the eNB of the target cell, where the confirm-message indicates that the communication between the user equipment and the target eNB can now resume. For example, the RRC connection re-establishment request, the re-establishment message, and subsequent confirm-message would typically include fields for supporting secure identification of the user equipment and fields for supporting contention resolution, i.e. such that the user equipment and its connection can be unambiguously and securely identified.
The recent and rapid uptake of Mobile Broadband has lead to a need for increasing the capacity of cellular networks. One solution to achieve such a capacity increase is to use denser networks consisting of several layers of cells with different sizes: Macro cells ensure large coverage with cells encompassing large areas, while micro-, pico- and even femto-cells are deployed in hot-spot areas where there is a large demand for capacity. Those cells typically provide connectivity in a much smaller area, but by adding additional cells and radio base-stations controlling those cells, capacity is increased as the new cells off-load the macros. Such networks are referred to as Heterogeneous Networks or HetNets. FIG. 9 shows a user equipment moving from the coverage of a pico-cell A into the coverage of macro cell B.
The different “layers” of cells can be deployed on the same carrier, i.e. in a reuse-1 fashion, the small cells could be deployed on a different carrier, and the different cells on the various layers could even be deployed using different technologies, e.g. 3H/HSPA on the macro- and micro-layer, and LTE on the pico-layer as one non-exclusive example.
There is currently a large interest for investigating the potential of such HetNets. However, it has also been found that HetNets may result in an increased rate of handover failures, as briefly discussed above. One reason is that the handover region in HetNets may be very short, meaning that the handover might fail since the user equipment lost coverage to the source cell before the handover to a target cell could be completed. For example, when a user equipment leaves a pico-cell, it may happen that the coverage border of the pico is so sharp, that the user equipment fails to receive any handover command towards a macro before loosing coverage to the pico, see FIG. 4.
Similar problems could occur when a user equipment connected to a macro cell suddenly enters a pico cell on the same carrier. It could now happen that the control channels of the pico interferes with the signals that the user equipment needs to receive from the macro in order to complete the handover, and the handover thus fails.
In failed handovers of the kind exemplified above, the user equipment will eventually try to re-establish the RRC connection. But this can only occur after the procedures prior to the recovery procedure have been completed, as described above: Thus, the user equipment will observe “out-of-sync” on Layer 1, L1, towards the source cell, those events will be counted on Layer 3, L3, level, i.e. RRC, as described above until N310 such consecutive events have occurred, and then T310 will be started. Only when T310 has expired, the user equipment can initiate re-establishment procedure by searching for a better cell to connect to, in order to recover the RRC connection.
It is clear that this counting (up to N310) and the waiting of T310 to expire will result in an undesired interruption of the connectivity that is likely to be observable for the end-user.
One could therefore argue that the network should configure the relevant counters and timers (with N310 and T310 as non-exclusive examples) with small values in order to speed up the recovery. However, this could result in premature loss of the connection in case the radio problems were not due to an imminent handover, but only due to a sudden fading dip.